"Ryan completely flipped out, and it got heated. Ryan got in the guy's face and said, 'Who are you calling baby?' Eva had to jump in and calm everyone down before it came to fisticuffs. Ryan then made nice and shook the guy's hand," an eye witness said.
The couple, who has been together since September 2011, are known to keep their relationship low-key and private.
In a party for their film The Place Beyond the Pines, Gosling and Mendes arrived at the hotel separately.
"I didn't see Eva and Ryan interact once. They stayed at opposite ends of the room from one another. They decided they were going to do what they needed to for the movie and get out as soon as possible. They didn't want to make a spectacle out of their relationship out of respect for Derek and his movie," a source said.